Carbonylation of alcohols and ethers to their corresponding esters is a well known art and is illustrated in Equations 1 and 2 below. EQU 2ROH+CO.fwdarw.RC(O)OR+H.sub.2 O (1) EQU ROR'+CO.fwdarw.RC(O)OR' (2)
In addition, alcohols can be carbonylated to carboxylic acids as shown in Equation 3, below. EQU ROH+CO.fwdarw.RC(O)OH (3)
Carbonylation of methanol, equations 1 and 3, is a well known reaction and is traditionally carried out in the liquid phase with a catalyst. The catalyst typically comprises a Group VIII metal, a halide promoter (normally some form of iodide) and occasionally a ligand such as PR.sub.3 or NR.sub.3 (R being organic moiety).
The most common catalyst for a liquid phase system is Rh in combination with HI/CH.sub.3 I. This catalyst is considered "state of the art" and is described in U.S. Pat. No. 3,769,329. This technology/catalyst is utilized in many commercial processes that generate acetic acid via methanol carbonylation. The operating conditions are about 180.degree.-200.degree. C. and about 450 psi CO with&gt;95% selectivity to methyl acetate/acetic acid.
Prior to the development of the Rh and HI/CH.sub.3 I system, the reaction was carried out in a liquid phase system with Co-I catalysts such as described in U.S. Pat. No. 3,060,233. The operating conditions are about 200.degree. C. and about 7,000-10,000 psi CO. The product selectivity is about 93%. A number of researchers have described the use of Ni-based catalysts for the carbonylation reaction. An example is EP 18 927. The catalyst of that publication consists of Ni and a combination of ionic iodides (example: KI) and covalent iodides (example: CH.sub.3 I). The reaction is carried out at about 150.degree. C. and about 800-1,000 psi CO.
All of the liquid phase catalysts that generate methyl acetate/acetic acid at commercially acceptable rates and selectivities require the use of an iodide promoter, typically CH.sub.3 I and/or HI, and high pressure (at least about 450 psi). The iodides are highly corrosive and necessitate the use of expensive corrosion resistant materials of construction. In addition, separation of the catalyst is a major problem and requires special equipment.
Vapor phase carbonylation of methanol has the advantage of easy product separation from the catalyst. In most cases, patents describing homogeneous catalysts also claim that the reaction can be carried out in the vapor phase [e.g., methanol and an iodide containing compound (CH.sub.3 I) are co-fed] with the catalyst supported on a material such as silica (SiO.sub.2) or alumina (Al.sub.2 0.sub.3). Rarely are examples given. The following descriptions refer to references which deal only with vapor phase carbonylation.
A considerable amount of work has been carried out on heterogeneous versions of the Rh/CH.sub.3 I system described above, see for example, Journal of Catalysis (13, p. 106, 1969 and 27, p. 389, 1972). In a typical illustration, Rh is supported on activated carbon and a gaseous mixture of CO, CH.sub.3 OH, and CH.sub.3 I is passed over the catalyst at 175.degree.-250.degree. C. and 300 psi. The catalyst is active and high yields of acetic acid/methyl acetate are obtained. In similar work, Rh is impregnated on Al.sub.2 O.sub.3 (Krzywicki et. al., Bull. Soc. Chim. France, 5, p. 1094, 1975), SiO.sub.2 and TiO.sub.2 (Krzywicki et. al., J. Mol. Cat., 6, p. 431, 1979) or zeolite-encapsulated (Schwartz et. al., J. Mol. Cat., 22, p. 389, 1984). In both cases CH.sub.3 OH and CH.sub.3 I are co-fed to the reactor containing the catalyst. It should be noted that in all these examples CH.sub.3 I is required in order for the carbonylation reaction to work.
In JA 59139330, the carbonylation catalyst consists of Ni supported on activated carbon. The reaction is carried out at 200.degree.-300.degree. C. and 150 psi with a feed mixture of CO:CH.sub.3 OH:CH.sub.3 I=5:1:0.01. At 300.degree. C., the methanol conversion is 100% and the acetic acid/methyl acetate selectivity is&gt;95%. In JA 59172436 the same catalyst is utilized to carbonylate dimethyl ether. In DE 3323654 a CH.sub.3 OH/CH.sub.3 I feed mixture is carbonylated with a Pd-Ni catalyst supported on activated carbon. The reaction is carried out at 300.degree. C. and 1 atm CO.
Gates, J. Mol. Cat., 3, p. 1, 1977, reports that Rh impregnated in crosslinked polystyrene is a vapor phase catalyst for the carbonylation of CH.sub.3 OH. Catalyst activity and stability are low. In JA 56104838 and JA 56104839 various Group VIII metals (Rh, Ni, Pd, Ru, Co) and rare earth metal oxides (Cs, La) are supported on silica. Methanol carbonylation is carried out at 150.degree.-200.degree. C. and 1-5 atm CO, and methyl acetate selectivity is high. In these examples, CH.sub.3 I is not utilized in the reaction feed.
Vapor phase processes that need CH.sub.3 I as a promoter will be corrosive and require expensive materials of construction. In addition, extensive separation/purification procedures are required in order to remove iodides from the product.
Heteropoly acids are well known compounds. The name "heteropoly acids" refers to a broad class of compounds of varying composition. A good general review of their physical properties is given by Tisgdinos in Climax Molybdenum Company, Bulletin Cdb-12a, 1969.
The use of heteropoly acids in many areas of catalysis is well known including dehydration of alcohols, Friedel-Crafts type reactions, oxidative dehydrogenation and partial oxidation of organic compounds. For examples see Matveev, et. al., J. Mol. Cat., 38, 345, 1986, Dun, et. al., Applied Catalysis, 21, 61, 1986, Nomiya, et. al., Bull. Chem. Soc. Jap., 53, 3719, 1980, and Izumi, et. al. J. Mol. Cat., 18, 299, 1983. Recently, much attention has been given to heteropoly acids as a catalyst for the conversion of methanol into hydrocarbons: EQU xCH.sub.3 OH.fwdarw.CH.sub.2 .dbd.CH.sub.2 +CH.sub.3 CH.dbd.CH.sub.2 +other hydrocarbons (4)
See Moffat, et. al., J. of Cat., 473, 1982, Ono, et. al. Bull. Chem. Soc. Jap., 55, p. 2657, 1982, and Moffat, et. al., J. of Cat., 81, p. 61, 1983. This reaction is carried out in the vapor phase (300.degree.-375.degree. C.) and the products include ethylene, propylene and saturated C.sub.1-5 hydrocarbons. Reaction 4 dominates the known chemistry of reactions of methanol in the presence of heteropoly acids. It was therefore unexpected to find that methanol carbonylation could be carried out with heteropoly acids.
Hetero polyacids and polyoxometalate anions constitute well recognized compositions. They embrace the well-known complexes called isopolyoxoanions and heteropolyoxoanions. They are represented by the general formulas.sup.1 FNT 1. See Pope, Heteropoly and Isopoly Oxometalates, Published by Springer-Verlag, Berlin, 1983, page 1.
______________________________________ [M.sub.m O.sub.y ].sup.p- Isopolyanions [X.sub.x M.sub.m O.sub.y ].sup.q- Heteropolyanions (x .ltoreq. m) ______________________________________
wherein M is at least one metal taken from Group V and VI of the Periodic Chart of the Elements (such as molybdenum, tungsten, vanadium, niobium, chromium, and tantalum) in their highest (d.sup.0, d.sup.1) oxidation states, and X is a heteroatom from all groups of the Periodic Chart of the Elements with the possible exception of the rare gases..sup.2 FNT 2. Pope, supra, page 2, takes the position that the "terms polyoxomealate or or polyoxoanion might be therefore more appropriate to describe the field."